Vapor generating system in which recirculated flue gases are injected into the combustion air stream of a steam generator

ABSTRACT

A vapor generating system in which a plurality of mixing pipes are disposed in a duct connecting the interior of the furnace section with a source of air. An additional duct introduces flue gases from the furnace section into the mixing pipes, and slots are provided through the mixing pipes for permitting the discharge of the gases into the path of air flowing through the air duct to mix the gases and air before they are introduced into the furnace section. A plurality of metering pipes are disposed in the air duct, and apparatus is provided which cooperates with the mixing pipes and the metering pipes to enable the quantities of gases and air to be respectively measured.

United States Patent Welden, Jr. et al. 1 July 1, 1975 [54] VAPOR GENERATING SYSTEM IN WHICH 2,229,643 1/1941 DeBaufre... 122/479 RECIRCULATED FLUE GASES ARE 2,926,636 3/1960 Paulison. .11. 122/479 INJECTED INTO THE COMBUSTION AIR A Prlmary Examiner-Kenneth Sprague Attorney, Agent, or Firm-Marvin A. Naigur; John E. [75] Inventors: Robert P. Welden, Jr., Westfield; Wilson; warren g Kice Walter P. Gorzegno, Florham Park. both of N1 57 ABSTRACT [73] Assignee2 9 wheeler Corporation A vapor generating system in which a plurality of mix- Lwmgstonv ing pipes are disposed in a duct connecting the inte- [22] Filed: May 13, 1974 rior of the furnace section with a source of air. An additional duct introduces flue gases from the furnace [21] PP N05 469,261 section into the mixing pipes, and slots are provided through the mixing pipes for permitting the discharge 52 us. Cl. 110/49 R; 122/235 B of the gases into the p of air flowing through the air 51 Int. cl. F23J 5/02 duct to mix the gases ahd air before y are intro- [58] Field of Seal-chm 1 0 49 22 4 9 A 235 B duced into the furnace section. A plurality of metering pipes are disposed in the air duct, and apparatus is [56] References Cited provided which cooperates with the mixing pipes and UNITED STATES PATENTS the metering pipes to enable the quantities of gases and air to be respectively measured. 1,789.401 1/1931 Davy 110/49 1.837.713 12/1931 Jacobus 110/49 12 Claims, 6 Drawing Figures I 1 III! I VAPOR GENERATING SYSTEM IN WHICH RECIRCULATED FLUE GASES ARE INJECTED INTO THE COMBUSTION AIR STREAM OF A STEAM GENERATOR BACKGROUND OF THE INVENTION This invention relates to a vapor generating system in which recirculated flue gases are injected into the com bustion air stream ofa steam generator, and the quantities of injected flue gases and combustion air are measured.

In the generation of heat in a furnace section of a vapor generator, a relatively high concentration of oxides of nitrogen are formed during combustion in the furnace and, when discharged therefrom, pollute the surrounding air. Also, when solid fuels are burned, severe slag deposits are often formed on the heat abosrption surfaces of the furnace section during operation thereof.

It has been discovered that the formation of nitric oxides and, in the case of solid fuels, slag deposits can be reduced by recirculating a portion of the flue gases from the convection section of the generator back into the furnace section, usually in a mixture with the combustion air. However, the resulting mixtures have been somewhat non-uniform and the mechanical arrangements utilized suffer from the standpoint that they are relatively bulky and often cause relatively high pressure losses, especially with respect to auxiliary devices utilized to measuse the recirculated gas and combustion air flow into the furnace section.

SUMMARY OF THE INVENTION It is, therefore, a general object of the present invention to provide an improved vapor generating system in which recirculated flue gases are injected into the combustion air stream in a manner to obtain a uniform mixture of the gases and air.

It is a further object of the present invention to provide an improved vapor generating system in which the respective flow rates of the recirculated flue gases and the combustion air entering the furnace section of the generator are obtained in an efficient manner requiring a minimum of additional duct work and a minimum of duct space.

Towards the fulfillment of these and other objects, the vapor generating system of the present invention includes duct means connecting the interior of a furnace section with a source of air and a plurality of mixing pipes disposed in said duct means. Flue gases are passed from said furnace section into said mixing pipes, with the mixing pipes having outlets for permitting the discharge of said gases into the path of air flowing through said duct means to mix said gases and air before they are introduced into said furnace section. A plurality of metering pipes are disposed in the air duct, and apparatus is provided which cooperates with the mixing pipes and the metering pipes to enable the quantities of gases and air to be respectively measured.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic view of an example of a vapor generator incorporating the system of the present invention;

FIG. 2 is an enlarged partial perspective view depicting a portion of a wall of the vapor generator of FIG. l;

FIG, 3 is a partial enlarged sectional view taken along the line 33 of FIG. 1;

FIG. 4 is an enlarged sectional view taken along the line 4l4l of FIG. ll;

FIG. 5 is an enlarged partial perspective view showing a section of the vapor generator of FIG. I; and

FIG. 6 is an enlarged, fragmentary sectional view of a portion of the arrangement of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. ll of the drawings, one arrangement of a vapor generator of the present invention is broadly indicated by the reference numeral MI and comprises a vertically extending rectangular-shaped radiant furnace portion 12 having a bottom hopper l4 and an upper outlet area 16 to which is connected a horizontally extending and downwardly extending convection section 1% having an outlet 20. As a result of this arrangement, hot gases flow upwardly in the furnace portion 12 through the outlet area l6, and downwardly through the convection section 1%, whereby they exit through the outlet 20 for further treatment in a manner to be described in detail later.

The furnace portion I2 is in the form of an upright rectangular enclosure extending vertically from the floor of the generator to a roof and defined by front and rear walls 24 and 26, and a pair of side walls, one of which is shown by the reference numeral 28.

A floor 30 connects the upper section of the furnace portion 12 to the convection section 118 with the latter being defined by a front wall panel 32, a rear wall panel 34, and a pair of side walls, one of which is shown by the reference numeral 36. It is understood that, in a typical arrangement, a vertical partition wall divides the convection section l8 into front and rear gas passes and that the convection section contains a plurality of superheater tubes, reheater tubes, and economizer tubes. The partition wall and tubes are not shown or described in the present invention since they do not form a part of the present invention, and since they are described in detail in US. Pat. No. 3,556,059, assigned to the assignee of the present invention, the disclosure of which is incorporated by reference.

FIG. 2 depicts the membranetype wall that is used throughout the generator, including the walls 24-, 26, 28, 32, 34, and 36, as well as the floor 30. In particular, the walls are formed by welding together a plurality of finned tubes 38 along their lengths so that the enclosures defined by the various walls are substantially gas tight. Of course, in a normal installation an insulating wall of a suitable insulating material would be provided adjacent the above-mentioned membrane-type walls, and externally thereof. For the convenience of presentation, these insulating walls have been omitted from the present drawings.

Water is passed through the tubes of each of the above-mentioned walls and is routed in a predetermined flow path by means of suitable headers, downcomers, etc., in a manner so that it is gradually turned into steam by virtue of the heat generated in the furnace portion l2, after which the steam is superheated and collected in a conventional manner.

Referring again to FIG. l, a duct 40 is connected to the convection section outlet 2% for receiving the flue gases passing from the convection section and directing a portion of same into an air heater 42. The air heater 42 is provided with an inlet 42a for receiving absorption from atmosphere, and an outlet 42b for passing the hot gases entering therein to a stack or the like, with the air heater providing a heat exchange between the gases and the air before they exit therefrom, in a conventional manner.

A wind box 46 is located at the lowerportion of the furnace portion 12 and supplies air, to the furnace portion 12 and typically includes front and rear plates 48 and 50 which are spaced from the corresponding portions of the front and rear walls 24 and 26, respectively, of the furnace portion. A plurality of burners 52 discharge through openings formed in the furnace walls 24 and 26. The details of the burners 52 will be described later. 7

A duct 56 connects the air heater 42 to the lower section of the wind box 46 for supplying the preheated air from the air heater 42 to the wind box 46.

A duct 60 connects the duct 40 to the lower portion of the duct 56 for supplying a portion of the hot flue gases from the convection section outlet 20 to the duct 56 under the force of a fan 62 disposed in the duct 60.

As a result, a mixing Zone, shown in general by the reference numeral 64 is defined in the duct 56 which mixes the combustion air from the preheater 42', and the recirculated flue gases from the duct 60. The details of the mixing zone 64 will be described later.

FIG. 3 depicts the burners 52 and their positions relative to the furnace portion 12 in detail. In particular, the burners are arranged in four vertical rows of four burners per row, it being understood that the number of burners and their particular pattern can vary. Since the burners 52 are of a conventional design and form no portion of the present invention, they are shown j only in general, with it being understood that they include an inlet for receiving fuel, which can be in liquid or gaseous form, or which can be a mixture of pulverized fuel and primary air. An annular space 68 is defined between the throat end of each burner 52 and its corresponding opening in the wall 24 for reasons that will be described in detail later. Since the arrangement of the other walls of the furnace portion 12 may be identical to that of wall 24 with respect to the disposition of the burners, they. will not be described in detail. As a result of the above arrangement, the air-gas mixture from the mixing zone 64 will be passed into the wind box 46, through the annular spaces 68, and into the furnace portion 12, as shown by the flow arrows in FIG. 3, to reduce the formation of nitric oxides and, in the event a solid fuel is used, to also reduce the slag deposits, as discussed above. It is understood that in some installations, overfire ports 100 may be provided through which a portion of the air-gas mixture would pass into the furnace portion 12. g

The details of the structure defining the mixing zone 64 as well as apparatus for measuring the fluid flow are discharged in two directions in the duct 56 from each pipe and perpendicular to the path of the air flowing through the duct 56. As a result, a relatively uniform mixture of gases and air is formed for passage into the wind box 46.

As stated earlier, it is necessary to determine the quantity of air flow into the wind box 46 and therefore into the furnace portion 12 through the burner throat annular openings 68 in order to properly maintain proper combustion of the fuelfTo this end, the velocity pressures of the air passing through the duct 56 and the static pressure difference of the recirculated flue gases passing through mix pipes 70 are obtained from which the air flow and gas flow rates can be determined.

To determine the velocity pressure of the air passing through theduct 56; one row of three metering pipes 74 are disposed in the inlet portion of the mixing zone 64 as shown in FIG. 5. Eachpipe 74 is closed at both ends and is provided with an opening 76 facing the upstream portion of duct 56 to measure total pressure of the air in the duct, which equals the velocity pressure plus the static pressure. An additional opening 78 is provided through each pipe 74 in a spaced relation to the opening 76 to measure static pressure of the air in the duct 56. A pair of tubes 80, and 82 each have an end portion extending within each pipe 74 and registering with the opening 76 and 78, respectively, with only the tubes associated with one of the pipes 74 being shown for the convenience of presentation. The tubes 80 and 82 are connected at their other ends to a flow meter 84 to transfer the respective pressures from the tubes to the flow meter. Since the pressure of the air in the tube 80 represents the total pressure of the air in the duct 56, i.e.. the velocity pressure plus the static pressure,

' and since the'pressure of the air in the tube 82 represents the static pressure of the air, it can be appreciated that the difference between the two pressures is equivalent to velocity pressure and that the flow meter 84 can, therefore, respond to this differential and provide a corresponding indication of the air flow. Since this is done in a conventional manner, the flow meter 84 is not shown in detail and will not be described in any further detail.

Since, in actual practice, all of the pipes 74 will be provided with the tubes 80 and 82, the connections will be such that the flow meter 84 will register a differential of the average pressures at the pipes 74. Also, it is understood that a plurality of openings 76 and 78 can be provided in each pipe 74 in which case the tubes 80 and 82 would be arranged and located to be in registry with all of the openings.

Referring specifically to FIGS. 5 and 6, one end portion of an exit tube 89 is connected to an openingprovided through at least one of the pipes 70 so that it communicates with the interior thereof, and a tube 90 is connected to the tube 89 in registry therewith. Also,

I an end portion of a tube 92 extends through an opening provided in the duct 60 and in the path of the recirculated gases in the latterduct. The other ends of the tubes 90 and 92 are connected to a flow meter 94, and since the pressure in the tube 92 represents the static pressure of the gases in the duct 60, and the pressure in the tube 90 represents the static pressure of the gases leaving the exit tube the flow meter 94 can be designed to provide an indication of the gas flow rate based on a difference in static pressures in a similar manner to that discussed above with respect to the flow meter 84.

It is understood that the tubes 89 and 90 can be provided at more than one of the pipes 70 and that a plurality of tubes 92 can be in communication with a plurality of openings formed in the duct 60. Also, the tubes 89 and 92 can be designed to register with a plurality of openings formed in the tubes 70 and the duct 60, re-

spectively. In the above situations, the flow meter 94 would respond to differences of average readings in determining the gas flow.

As a result, the arrangement of the present invention enables the recirculated flue gases to be uniformly mixed with the combustion air and utilized to reduce the formation of nitric oxides and, when solid fuels are used, to also reduce slag deposits. In addition, the present arrangement provides for flow measurements of the recirculated flue gases and the combustion air, while reducing the ducting requirements and the pressure losses when compared to those arrangements using oriflce meters, air foils, venturis and the like.

It can be appreciated that the above arrangement of the furnace and convection sections of the vapor generator of the present invention has been described only by way of example, and that other arrangements are equally applicable. Also, other variations of the arrangement disclosed above can be made by those skilled in the art without departing from the invention as defined in the appended claims.

What is claimed is:

1. A vapor generating system comprising a furnace section, duct means connecting the interior of said furnace section with a source of air, a plurality of mixing pipes disposed in said duct means, and additional duct means for introducing combustion gases from the outlet of said furnace section into said mixing pipes, said mixing pipes having a plurality of slots extending therethrough for discharging said gases into the path of air flowing through said duct means in a direction perpendicular to the direction of the flow of said air to mix said gases and air before they are introduced into said furnace section.

2. The generating system of claim 1 wherein said slots are adapted to discharge said gases in a transverse direction across said duct means.

3. The generating system of claim 1 wherein a plurality of openings are provided through at least one of the walls of said furnace for permitting the mixture of gases and air to enter said furnace section and further comprising a plurality of burners mounted relative to said wall for discharge through said openings.

4. A vapor generating system comprising a furnace section, duct means connecting the interior of said furnace section with a source of air, a plurality of mixing pipes disposed in said duct means, additional duct means for introducing combustion gases from the outlet of said furnace section into said mixing pipes, said mixing pipes having outlets for permitting the discharge of said gases into the path of air flowing through said duct means to mix said gases and air before they are introduced into said furnace section, said additional duct means and at least one of said mixing pipes having an opening formed therein, and means registering with said openings and responsive to the respective gas pressures at the openings for enabling the flow rate of said gases through said duct means to be determined.

5. The generating system of claim 4 wherein said means registering with said openings comprises a tube having one end registering with one of said openings, a tube having one end registering with the other of said opening, and a flow meter connected to the other ends of said tubes and responsive to the respective fluid pressures in said tubes.

6. A vapor generating system comprising a furnace section, duct means connecting the interior of said fur nace section with a source of air, at least one metering pipe disposed in said duct means and having at least one opening in the direct path of air flow through said duct means and at least one additional opening spaced from said one opening out of the direct path of air flow through said duct means, and means registering with said openings and responsive to the respective air pressures at the openings for enabling the flow rate of said air through said duct means to be determined.

7. The generating system of claim 6 wherein said means registering with said openings comprises a tube having one end registering with one of said openings, a tube having one end registering with said additional opening and a flow meter connected to the other ends of said tubes and responsive to the respective fluid pressures in said tubes.

8. The generating system of claim 6 further comprising a plurality of mixing pipes disposed in said duct means, and additional duct means for introducing combustion gases from said furnace section into said mixing pipes, said mixing pipes having outlets for permitting the discharge of said gases into the path of air flowing through said duct means to mix said gases and air before they are introduced into said furnace section.

9. The generating system of claim 8 wherein a plurality of openings are provided through at least one of said walls for permitting the mixture of gases and air to enter said furnace section and further comprising a plurality of burners mounted relative to said wall for discharge through said openings.

10. The generating system of claim 8 wherein said outlets are in the form of a plurality of slots extending through said mixing pipes and adapted to discharge said gases in a direction perpendicular to the direction of said air flow.

ll 1. The generating system of claim 8 wherein said additional duct means and at least one of said mixing pipes has an opening formed therein and further comprising means registering with said latter openings and responsive to the respective gas pressures at the openings for enabling the flow rate of said gases through said duct means to be determined.

12. The generating system of claim llil wherein said means registering with said latter openings comprises a tube having one end registering with one of said latter openings, a tube having one end registering with the other of said latter opening, and a flow meter connected to the other ends of said tubes and responsive to the respective fluid pressures in said tubes. 

1. A vapor generating system comprising a furnace section, duct means connecting the interior of said furnace section with a source of air, a plurality of mixing pipes disposed in said duct means, and additional duct means for introducing combustion gases from the outlet of said furnace section into said mixing pipes, said mixing pipes having a plurality of slots extending therethrough for discharging said gases into the path of air flowing through said duct means in a direction perpendicular to the direction of the flow of said air to mix said gases and air before they are introduced into said furnace section.
 2. The generating system of claim 1 wherein said slots are adapted to discharge said gases in a transverse direction across said duct means.
 3. The generating system of claim 1 wherein a plurality of openings are provided through at least one of the walls of said furnace for permitting the mixture of gases and air to enter said furnace section and further comprising a plurality of burners mounted relative to said wall for discharge through said openings.
 4. A vapor generating system comprising a furnace section, duct means connecting the interior of said furnace section with a source of air, a plurality of mixing pipes disposed in said duct means, additional duct means for introducing combustion gases from the outlet of said furnace section into said mixing pipes, said mixing pipes having outlets for permitting the discharge of said gases into the path of air flowing through said duct means to mix said gases and air before they are introduced into said furnace section, said additional duct means and at least one of said mixing pipes having an opening formed therein, and means registering with said openings and responsive to the respective gas pressures at the openings for enabling the flow rate of said gases through said duct means to be determined.
 5. The generating system of claim 4 wherein said means registering with said openings comprises a tube having one end registering with one of said openings, a tube having one end registering with the other of said opening, and a flow meter connected to the other ends of said tubes and responsive to the respective fluid pressures in said tubes.
 6. A vapor generating system comprising a furnace section, duct means connecting the interior of said furnace section with a source of air, at least one metering pipe disposed in said duct means and having at least one opening in the direct path of air flow through said duct means and at least one additional opening spaced from said one opening out of the direct path of air flow through said duct means, and means registering with said openings and responsive to the respective air pressures at the openings for enabling the flow rate of said air through said duct means to be determined.
 7. The generating system of claim 6 wherein said means registering with said openings comprises a tube having one end registering with one of said openings, a tube having one end registering with said additional opening and a flow meter connected to the other ends of said tubes and responsive to the respective fluid pressures in said tubes.
 8. The generating system of claim 6 further comprising a plurality of mixing pipes disposed in said duct means, and additional duct means for introducing combustion gases from said furnace section into said mixing pipes, said mixing pipes having outlets for permitting the discharge of said gases into the path of air flowing through said duct means to mix said gases and air before they are introduced into said furnace section.
 9. The generating system of claim 8 wherein a plurality of openings are provided through at least one of said walls for permitting the mixture of gases and air to enter said furnace section and further comprising a plurality of burners mounted relative tO said wall for discharge through said openings.
 10. The generating system of claim 8 wherein said outlets are in the form of a plurality of slots extending through said mixing pipes and adapted to discharge said gases in a direction perpendicular to the direction of said air flow.
 11. The generating system of claim 8 wherein said additional duct means and at least one of said mixing pipes has an opening formed therein and further comprising means registering with said latter openings and responsive to the respective gas pressures at the openings for enabling the flow rate of said gases through said duct means to be determined.
 12. The generating system of claim 11 wherein said means registering with said latter openings comprises a tube having one end registering with one of said latter openings, a tube having one end registering with the other of said latter opening, and a flow meter connected to the other ends of said tubes and responsive to the respective fluid pressures in said tubes. 